This invention relates to a system and a method for synchronizing the phase angle for an alternating current (xe2x80x9cACxe2x80x9d) power source which operates in parallel with a power grid, and more particularly, to a system and a method which provides reliable synchronization of the phase angle for an AC power source operating in parallel with a grid in both normal and abnormal operating conditions.
Alternating current power sources (xe2x80x9cAPSxe2x80x9d), such as static power converters (xe2x80x9cSPCxe2x80x9d), are commonly used to provide electrical energy and power to electrical devices, networks or grids. One type of APS, commonly referred to as a three-phase AC power source, provides voltage in three separate phases. A requirement for this type of three-phase power delivery arrangement is that the voltage phase angle of the APS is synchronized with the voltage phase angle of the grid.
The phase synchronization of an APS and its associated grid is typically controlled through the use of one or more xe2x80x9cphase lock loopsxe2x80x9d (xe2x80x9cPLLxe2x80x9d). While the PLL technique can maintain synchronization between the APS and grid during many operating conditions, it suffers from several drawbacks.
For example and without limitation, in abnormal operating conditions, if grid voltage is lost for a short duration of time (e.g., for 0.1 seconds, as a result of switch xe2x80x9cbouncexe2x80x9d), the PLL will not function properly, as it will not receive critical inputs from the grid. Under such fault conditions, the PLL will lose synchronization with the grid and abnormal current may undesirably occur (e.g., over current, distortion current and other forms of disruptive current). Furthermore, even very small errors in synchronization, if not properly corrected or compensated, will accumulate over time causing the creation of relatively large undesirable errors. Such errors and corresponding abnormal currents often prevent reliable parallel operation between the APS and grid and may damage devices or components within the system.
There is therefore a need for a system and a method for providing relatively precise synchronization between an APS and a power grid, and which is effective to reliably generate the synchronized phase angle for the APS to connect to the grid during both normal and abnormal operating conditions.
It is a first object of the invention to provide a system and a method for synchronizing the phase angle for an alternating current power source (xe2x80x9cAPSxe2x80x9d) operating in parallel with a power grid which overcomes some or all of the previously delineated drawbacks associated with prior systems and methods.
It is a second object of the invention to provide a system and a method for synchronizing the phase angle for an APS operating in parallel with a power grid, which provides reliable synchronization during both normal and abnormal operating conditions.
It is a third object of the invention to provide a system and a method for precisely synchronizing the phase angle for a three-phase APS operating in parallel with a power grid.
According to a first aspect of the present invention, a system is provided for synchronizing a first phase angle of a power source with a second phase angle of a power grid. The system includes a first portion which receives a grid voltage and which uses the grid voltage to calculate the second phase angle; a second portion which is communicatively connected to the first portion, which receives the second phase angle, and which uses the second phase angle to calculate a grid voltage frequency; a third portion which is communicatively connected to the second portion, which receives the grid voltage frequency, and which uses the grid voltage frequency to generate an increment angle; a fourth portion which is communicatively connected to the first portion, which receives the second phase angle, and which generates an initial angle based upon the second phase angle; and a fifth portion which is communicatively connected to the third and fourth portions, which selectively receives the increment angle and the initial angle, and which uses the received increment and initial angles to generate the first phase angle having a value that is substantially identical to the second phase angle.
According to a second aspect of the present invention, a method is provided for synchronizing a first phase angle of a power source with a second phase angle of a power grid. The method includes the steps of receiving a grid voltage from the power grid; calculating the, second phase angle by use of the grid voltage; calculating a grid voltage frequency by use of the second phase angle; generating an increment angle by use of the grid voltage frequency; generating an initial angle based upon the second phase angle; and using the increment and initial angles to generate the first phase angle having a value that is substantially identical to the second phase angle.
These and other features, advantages, and objects of the invention will become apparent by reference to the following specification and by reference to the following drawings.